beneath the table to the abstract. Use the Word Count cmd under Utilites menu for counting Significance Vaccination of HIV-infected pregnant women has the potential to prevent HIV transmission from a mother to her infant by lowering maternal virus load and/or by enhancing the levels or the quality of antiviral antibodies transferred to the fetus transplacentally. Vaccination may be less costly than antiviral drugs and thus, more economically feasible to use in developing nations. Objectives Infection of newborn rhesus macaques with simian immunodeficiency virus is a well-established model for pediatric HIV infection and AIDS. This study evaluated whether heat-inactivated rhesus serum containing high levels of SIV envelope-specific antibodies or HIV envelope-specific antibodies injected subcutaneously into newborn rhesus macaques could protect against oral SIV infection. Results Two rhesus newborns received SIVmac251 hyperimmune serum and four newborns received SHIV-33 (virus containing the HIV-sf33 envelope in an otherwise SIV genome) hyperimmune serum. Infants were inoculated orally 48 hours later with a high dose of pathogenic SIVmac251. This oral dose of SIV infected four naive neonatal rhesus; all four developed high virus levels in peripheral blood and fatal immunodeficiency by 12-26 weeks pi. In contrast, only one of the two neonates with SIV-specific antibodies was infected after oral SIV challenge and developed fatal simian AIDS. Thus, passive immunization of newborns with hyperimmune protected only 2 of six infants against oral SIV infection, but did not delay disease in the neonates that became SIV-infected. These results suggest that an HIV vaccine that could generate high levels of anti-HIV antibodies in HIV-infected pregnant women might reduce the risk of HIV transmission to infants during the birth process or by breast-feeding. Future Directions Immunogenicity of anti-SIV vaccine regimens will be compared in rhesus newborns that have (1) no anti-SIV antibodies, or (2) passively acquired anti-SIV antibodies to determine whether passive antiviral antibodies interfere with neonatal vaccination against SIV. KEY WORDS pediatric simian AIDS, oral SIV transmission, perinatal HIV transmission, passive immunization FUNDING NIH Grants RR00169 and AI039109, E. Glaser Pediatric AIDS Foundation Scientist Award 8-97